Conventional practice for locating underground utilities involves a manual, audio technique where a technician using a hand-held sensor with audio feedback first assumes where the target utility is located so that he can place a sensor on the assumed location to manually track the location of a utility based upon the audio feedback. The technician then makes paint marks on the ground over the underground utility so that this location can be subsequently mapped and so that an excavator can see the expected location of the utility.
This manual audio technique is subject to a number of problems. It requires a knowledgeable operator capable of assuming generally where a target utility is located so that a transmitter can be placed on the target utility and then used to manually follow the feedback signal. Since at least an assumption of utility location is required, unexpected utilities are routinely missed.
An operator needs to be able to sort-out conflicting audio signals in complicated environments, and still errors can be made as signals jump from one utility to the next. With this technique, utility depth information is so inaccurate that it is usually not recorded.
In more recent cable locating systems, the operator uses a conventional cable locator to find a cable and then, instead of paint, uses (Global Positioning System) GPS data to mark the location.
The operator, by interpretation of cable location, attempts to manually locate the cable locator over the cable. Then as the operator manually attempts to track along the cable using the cable locator, he periodically marks the location of the cable locator using the GPS system to thereby record the cable location. This technique, which is disclosed by U.S. Pat. No. 5,576,973 to Haddy, is subject to the difficulties previously enumerated for similar manual techniques. Only a single cable within an area is tracked, and accuracy is predicated upon the operator's interpretation as to where the underground cable may be located. Also GPS data alone may not be sufficiently accurate to track a small underground utility, such as a cable, without additional data from a stationary receiver.